Water detection methods are available for a variety of end use applications. For example, the water content of porous materials like soil has been measured by using electromagnetic fields, as described in U.S. Pat. No. 5,442,293. Moreover, the determination of moisture content in materials like wet- or cured concrete is sometimes very important in the building industry. U.S. Pat. No. 3,870,951 describes an electrical measuring probe useful for such a purpose.
Water detection is also a critical task when water-cooled electrical generators are employed. The stator yoke in these generators surrounds the armature core and partially encloses the armature windings, which are sometimes referred to as "stator windings" or "stator bars". As one typical example, copper conductors are usually wound in the armature to form loops. The armature windings are arranged in such a manner that the desired voltage and current characteristics can be maintained by the generator in operation. (Current usually flows through the stator in three phases). A number of the individual conductors (sometimes referred to herein as "strands") inside the stator bars are hollow, to allow for the flow of cooling water from a coolant system.
Electrical insulation is wrapped around both the strands and the stator bars, and is also often used to separate some of the strands from each other, or from other conductive structures, e.g., portions of the stator yoke which is usually made of steel. The ground wall insulation which is usually wrapped around the stator bars can be formed of various materials. Examples are fiberglass tape, vacuum/pressure-impregnating resins, casting and potting resins, and different types of laminates made by bonding layers of a reinforcing web.
Mica-based insulating tapes are often used in generators and large motors for a number of reasons. For example, these types of materials provide insulation of high electrical strength and excellent resistance to partial discharges. These materials also perform well in a high temperature environment. Various types of mica-based tapes are available (e.g., Micapal.TM. tapes). Most of them consist of mica flakes or laminates bound together with a resinous binder, such as an epoxy material. Prior to being cured, the material is flexible enough to be wrapped around a conductive element. When cured, the resulting material is thin and tough.
The durability and integrity of the insulation during operation of electrical generators is of great importance. The stator bars operate at very high voltages, e.g., greater than 10,000 volts in a large generator. The voltage has to remain isolated from ground. Any "flashover" from one stator bar to another, or from one electrical phase to another, could activate safety mechanisms which automatically shut down the generator. A sudden shut-down could instantaneously direct the current flow (often greater than 2,000 amps) to ground--an event which in some circumstances could severely damage many of the generator components.
As those who maintain water-cooled electric generators are well-aware, the leakage of water into the ground-wall insulation can damage it and ultimately lead to the catastrophic failures mentioned above. Water leaks from the coolant system are often found in or near the many brazed connections at the junction of a stator winding terminus and a water hose connection. The leaks are caused by a variety of occurrences, e.g., stress cracks or porosity in copper castings; or corrosion of the braze materials. As described by J. Timperley in Rotating Machinery, 62 PAIC 95 (copyright 1995 Doble Engineering Co.), water can then begin migrating along voids between the ground wall insulation and the strands, and can delaminate the mica-flake tape layers within the ground wall insulation. Failure of the generator can occur when water contaminates the ground-wall insulation in the vicinity of the stator core, where higher voltage stresses are present. Although on-line failures of generators due to water leakage are a rare occurrence, the damage caused by such an event could be extreme, as mentioned above.
Failure (i.e., according to test specifications) is most often experienced during routine maintenance or testing of the generator. For example, a stator water pumping unit may be left in operation when the generator is degassed. Under those conditions, the pressure differential may force water through a leak site and into the ground-wall insulation. In general, even very small water leaks can be detrimental to a generator if they are allowed to persist.
There are a number of techniques which are presently used to detect water in electrical insulation. Capacitance mapping is a popular technique, and is described in the Timperley article mentioned above, as well as in other references. In most variations of this technique, an electrode is brought into contact with the insulation, forming a type of capacitor when a DC (direct current) potential is applied across the insulation. As a specific example, a conductive pad could be placed on each stator bar in its end-arm region, and capacitance-to-ground is recorded and compared with readings from adjacent stator bars. The dielectric constant of insulation increases with water content therein, leading to a higher capacitance reading as compared to readings for dry insulation. A plot of the readings can be made, and certain capacitance values or deviations from other values (or from a mean value) can be designated as failures, based on the plot values.
While capacitance mapping is useful in some situations, it also has disadvantages. The technique is often not especially sensitive, with data variations of greater than about 10%. Such a variation requires even larger deviations (e.g., 20%-25%) for particular readings to be meaningful. In such an instance, allowances in regard to the failure-threshold can result in a significant number of passable insulation sections registering as failures. Moreover, thickness variations in the insulation for different stator bars may lead to differences in capacitance measurements for materials having the same water content, or having no water content. This type of variation makes relative comparisons of capacitance readings difficult. Furthermore, the composition of the insulator may adversely affect capacitance mapping. The technique relies on water having a large dielectric constant as compared to the insulating material. However, if the insulator contains inorganic fillers or other constituents, its dielectric constant may be increased to a level closer to that of water, making comparative measurements more difficult.
Perhaps the most serious drawback associated with capacitance mapping is the need for the generator to be off-line when the technique is being used. Usually, the generator must be disconnected from power transmission systems, and the electrical phases must be isolated. The time and labor required in bringing the generator off-line can represent a considerable expense for utility companies or other entities which generate electricity.
One on-line technique for water detection is known as the stator leak monitoring system (SLMS). Such a system relies on a generator arrangement in which the stator is sealed to prevent the entry of air, and is pressurized with hydrogen. The hydrogen pressure is maintained above the pressure of the water in the coolant system. In the event of a water leak, hydrogen will flow into the coolant system and be detected.
Although SLMS can advantageously be employed while the generator is operating, its use is also accompanied by some disadvantages. For example, the technique does not directly measure the presence of water. Instead, it provides an indication that a hydrogen leak is present somewhere, but it does not tell the operator where water leaks may be occurring. It also does not provide an indication that any of the insulation is in fact wet, or where the site of wetness might be.
A direct current technique for detecting water "trees" in insulated power cables has been described by H. Oonishi et al in the literature (IEEE Transactions on Power Delivery, Vol. PWRD-2, No. 1, January 1987). The method involves inserting a probe (which is part of an electrical measuring circuit) into the insulation. The probe is designed to detect any flow of direct current, which in turn serves as an indication of the existence of a water tree.
While the DC technique may be useful in some instances, it also is accompanied with some disadvantages. For example, the probe may not be especially sensitive, because it only detects water which is physically present in the free state on the surface of the probe. Moreover, to be effective, the probe must be physically inserted into the insulation, or encapsulated around it. Such a requirement can cause difficulty if the probe is used for the on-line testing of generators. Moreover, insertion of a probe after the generator has already been installed at a site could compromise the integrity of the insulation.
Other techniques for detecting water links in various settings are also available, e.g., the use of a tracer gas such as sulfur hexafluoride, or the use of thermographic video cameras. However, each technique is characterized by at least one of the disadvantages noted above. The disadvantages can represent special difficulties when the insulation being monitored is part of a high-voltage electrical generator.
Thus, one can readily understand that new methods for detecting the presence of water in materials would be of considerable value. The methods should be capable of accurately detecting water in the surface region of a variety of materials which have an affinity for water molecules. Moreover, these new techniques should be suitable for an insulating material which is incorporated into electrical power equipment, e.g., water-cooled generators. It would be especially beneficial if the techniques could be employed while the power equipment was in operation, so that unnecessary shut-downs could be avoided. Finally, the new methods should be relatively cost-effective, and not add significant expense to any of the related procedures, like power generation.